1. Field of the Invention
The present invention relates generally to a valve timing control system for an internal combustion engine, and more specifically, to such a valve timing control system that can adjust a phase relationship of opening and closing timings between an intake valve and an exhaust valve so as to control a magnitude of valve overlap where both the intake valve and the exhaust valve are opened, according to an engine driving condition.
2. Description of the Background Art
A valve timing control system for an internal combustion engine is known, such as disclosed in Japanese Patent First Publication No. 62-191636, wherein a phase relationship of valve timings (opening and closing timings) between an intake valve and an exhaust valve is variable so as to control a magnitude of valve overlap (both the intake and exhaust valves being opened) to be optimum depending on a monitored engine operating condition.
In this publication, the valve timing control system includes a valve timing varying mechanism. The valve timing varying mechanism includes a timing pulley which is driven by an engine crankshaft in synchronism therewith through a timing belt, a camshaft for actuating an intake valve, and a valve timing adjusting element provided between the timing pulley and the camshaft. Gear means is further provided between the timing pulley and the valve timing adjusting element and between the valve timing adjustment element and the camshaft such that an axial movement of the valve timing adjusting element causes a rotational displacement of the camshaft relative to the timing pulley, i.e. the engine crankshaft to change an angular phase relationship therebetween. Accordingly, when the valve timing adjusting element moves axially to rotate the camshaft relative to the engine crankshaft, the valve timing of the intake valve is varied relative to that of the exhaust valve so that variation of a magnitude of the valve overlap is caused.
The valve timing control system further includes a hydraulic circuit which selectively supplies a preset hydraulic pressure to the valve timing varying mechanism. The hydraulic circuit includes a pressure control valve in the form of a solenoid valve which is energized to close so as to establish the hydraulic pressure within the hydraulic circuit to move the valve timing adjusting element axially in one direction against a force of a coil spring to rotate the camshaft in one direction so as to advance the valve timing of the intake valve to enlarge the valve overlap, the solenoid valve is selectively deenergized to open so as to release the hydraulic pressure therethrough to allow the valve timing adjusting element to move in an axially opposite direction to an initial position by means of the spring force so as to rotate the camshaft in an opposite rotational direction. In the initial position of the valve timing adjust element, the phase relationship of the valve timings between the intake valve and the exhaust valve is initialized where the valve overlap is set minimum.
The valve timing control system further includes a control unit which controls the energization of the pressure control valve according to the engine driving condition. Specifically, when the engine is operated at a low or intermediate speed under an engine high load, the control unit energized the pressure control valve to enlarge the valve overlap. As is well known in the art, under these conditions, it is desirable to enhance an intake efficiency or a volumetric efficiency of air/fuel mixture into an engine combustion chamber using the intake inertia as much as possible to improve the engine torque. Accordingly, the valve overlap is set larger as noted above. On the other hand, when the engine is operated at the low or intermediate speed under an engine low load, the control unit deenergizes the pressure control valve so as to make the valve overlap smaller. Specifically, when the valve overlap is large under these engine driving conditions, a large amount of the exhaust gas runs back into the engine combustion chamber drawn by a large intake vacuum within the engine combustion chamber. Generally, since less fuel is supplied under the engine low load, the air/fuel mixture to be combusted in the combustion chamber includes an insufficient amount of fuel while a large amount of the residual combustioned gas is present, resulting in poor combustion and leading to an insufficient engine torque which becomes serious, particularly when the engine speed is low. Accordingly, the valve overlap is set smaller as noted above. Further, when the engine is operated at speed which is higher than a certain high speed, the control unit deenergizes the pressure control valve to set the valve overlap smaller since the large valve overlap reduces the maximum engine torque under such a high engine speed even when the engine load is high.
In the background art as noted above, however, since the magnitude of the valve overlap is controlled solely by the engine speed and the engine load, the following defects have arisen:
For example, in an automatic drive car, when an accelerator pedal is immediately released after a so-called stall starting where the accelarator pedal is applied to increase the engine speed with a brake pedal being also applied so as to start the car with the increased engine speed, an engine stall or stop, or an extreme engine speed reduction is caused. Specifically, since the engine speed and the engine load are increased in the stall starting, the control unit energizes the pressure control valve to enlarge the valve overlap as noted above. When the accelarator pedal is suddenly released under this condition, the control unit stops the energization to the pressure control valve for setting the valve overlap smaller. Since there exists a considerable delay or lag until the valve timing adjusting element actually returns to its initial position due to the displacement of the valve timing adjusting element being controlled by the hydraulic pressure, the valve overlap cannot be made smaller immediately after the accelerator pedal is released. Accordingly, even when the engine speed is lowered to near an engine idling speed, the magnitude of the valve overlap does not return to its predetermined initial minimum value due to the response delay of the valve timing adjusting element so that the air/fuel mixture becomes too lean to produce an engine torque large enough to maintain the engine operation by itself.